The channel estimation task for multi-antenna OFDM systems in a high mobile environment is a challenging one. Due to the rapid change of the channel environment, frequent update of the channel estimation is required for reliable data detection. For future OFDM systems, the length of a physical data packet and of an OFDM symbol will be increased such that one-time channel estimation through training sequences won't adequately represent the channel variation influenced by the whole packet under even a moderate mobile channel environment.
In the field of Orthogonal Frequency Division Multiplexing (OFDM) systems, the conventional multi-antenna OFDM channel estimation requires the attachment of a known training sequence to the physical data packet before its transmission, thus reducing overall bandwidth efficiency. Also, the conventional multi-antenna channel estimation requires complex computation, usually involving a computationally intense matrix inversion calculation. In a high Doppler channel environment where the channel changes rapidly, it means frequent updates of the channel estimation for reliable data detection, consequently incurring further loss of bandwidth and increase of computational loads for the conventional Multiple Input Multiple Output (MIMO) channel estimation.
This issue will be more critical for future OFDM systems such as 4G where the size of an OFDM symbol and a physical data packet is expected to be long. (For example, an OFDM symbol with 2048 subcarriers and a physical data packet near 24 OFDM symbols long are expected.) Therefore, computationally simple yet effective channel estimation for multi-antenna OFDM system has been the goal of many research activities.
Fast fading caused by a fast moving user terminal in cellular systems causes time variation of the Channel Impulse Response (CIR) over the physical OFDM frame. That variation in turn causes detection errors when using the FT in the receiver. A frame typically consists of a large number of OFDM symbols for the future 4G system, but there is only one pilot preamble to the frame used for synchronization and channel estimation. As the symbol number in the frame advances, the CIR varies and the CIR obtained from the preamble may become inaccurate. The higher the maximum Doppler frequency is, the more rapidly outdated the CIR obtained from the pilot preamble will be.
Recently, a similar channel estimation method has been proposed to place pilot symbols every M data symbols, thereby enabling a re-estimation of the CIR (G. Zhou, et al. “A First Order Statistical Method for Channel Estimation”, IEEE Trans. Signal Proc. Letters, Vol. 10, No. 3, March 2003, pp. 57-60. However, no further investigation on the multiple antenna OFDM system has been provided. In addition, no impulse train design issue has been mentioned.
Another proposal (J. C. Olivier, Derivative Equalization, May 2002, 4G Project Database) was to use a so-called derivative equalizer, a device that equalizes the time derivative of the CIR, rendering the frame time invariant. However, the equalizer is linear and causes noise enhancement.